Nematodes are elongated symmetrical roundworms that constitute one of the largest and most successful phyla in the animal kingdom. Many nematode species are free-living and feed on bacteria, whereas others have evolved into parasites of plants and animals, including humans. Human infections with parasitic nematodes are among the most prevalent Infections worldwide. Over one billion people, predominantly in tropical and subtropical developing countries, are infected with soil and vector-borne nematodes that cause a variety of debilitating diseases (Liu L. X., Weller P. F., Intestinal Nematodes. Chapter 181, in Harrison's Principles of Internal Medicine, Isselbacher K J, Braunwald E, Wilson J D, Martin J B, Fauci A S, Kasper D L, eds., New York: McGraw-Hill, pp. 916–920, 1994). Among these parasitic nematodes are Ancylostoma and Necator hookworms that cause anemia and malnutrition, Ascaris roundworms that can cause pulmonary and nutritional disorders, and Strongyloides stercoralis which can cause potentially lethal hyperinfection in immunocompromised patients (Liu L. X., Weller P. F., Strongyloidiasis and other intestinal nematode infections, Infect Dis Clin N Am 7, 655–682, 1993). Nematodes of the order Spirurida are responsible for onchocerciasis (river blindness) and lymphatic filariasis. Animal parasitic nematodes infect a wide variety of both domestic and wild animals. Major animal pathogens include Haemonchus contortus, which infects herbivorous vertebrates, Trichinella spiralis, the causative agent of trichinosis, and various members of the order Ascaridida, which infect pigs and dogs in addition to humans.
Plant parasitic nematodes also represent major problems, being responsible for many billions of dollars in economic losses annually. The most economically damaging plant parasitic nematode genera belong to the family Heterderidae of the order Tylenchida, and include the cyst nematodes (genera Heterodera and Globodera) and the root-knot nematodes (genus Meloidogyne). The soybean cyst nematode (H. glycines) and potato cyst nematodes (G. pallida and G. rostochiensis) are important examples. Root-knot nematodes infect thousands of different plant species including vegetables, fruits, and row crops. In contrast to many viral and bacterial pathogens, little is known about the molecular basis of nematode parasitism, limiting the available framework for rational anthelminthic (anti-nematode) drug development (David J. R., Liu L. X., Molecular biology and immunology of parasitic infections, Chapter 170 in Harrison's Principles of Internal Medicine, Isselbacher K J, Braunwald E, Wilson J D, Martin J B, Fauci A S, Kasper D L, eds., New York: McGraw-Hill, pp. 865–871, 1994).
Anti-nematode drug or pesticide discovery has traditionally relied either on direct screening of compounds against whole target organisms or on chemical modification of existing compounds, strategies that have resulted in relatively few classes of active agents acting against a limited number of known biological targets. For example, organophosphates and carbamates, the oldest extant class of nematicides, were developed many decades ago and target a single, biologically conserved enzyme, acetylcholinesterase. Imidazole derivatives such as benzimidazole exert their antiparasitic effects by binding tubulin. Levamisole acts as an agonist on the nicotinic acetylcholine receptor, and avermectins act as irreversible agonists at glutamate-gated chloride channels (Liu L. X., Weller P. F., Drug Therapy: Antiparasitic Drugs, N Engl J Med 334, 1178–1184, 1996). Unfortunately, there are certain debilitating nematode infections which are difficult if not impossible to cure with existing therapeutics. For example, in onchocerciasis, the adult female Onchocerca volvulus worms are refractory to even newer generation drugs (Liu L. X., Weller P. F., Drug Therapy: Antiparasitic Drugs, N Engl J Med 334, 1178–1184, 1996). In addition, drug resistance has emerged to all of these main classes of therapeutics, particularly in livestock animal applications in which their use is widespread (Sangster N. C., Gill J., Pharmacology of anthelminthic resistance, Parasitol Today 15, 141–146, 1999). Thus far it has not been possible to develop effective and practical vaccines, and even if such vaccines become available, effective anti nematode drugs will still be needed for treating established infections as well as offering the potential advantages prophylaxis and treatment for a broad spectrum of nematode parasites.
The drawbacks of existing agents that are currently used to control plant parasitic nematodes are equally or more significant. Fumigant nematicides such as methyl bromide and 1,3-dichloropropene, which kill nematodes by slowly diffusing through the soil, are phytotoxic and must be applied well before planting. Environmental concerns, primarily groundwater contamination, ozone depletion, and pesticide residues in food (National Research Council, Pesticides in the Diet of Infants and Children (Washington, D.C.: National Academy of Sciences, 1993) have prompted the removal of Aldicarb, DGBCP, and other toxic nematicides from the market by the Environmental Protection Agency, with methyl bromide to be withdrawn in the U.S. by 2002 (Johnson, S. L., Bailey, J. E., “Pesticide Risk Management and the United States Food Quality Protection Act of 1996”, in Pesticide Chemistry and Bioscience: The Food-Environment Challenge, Brooks, G. T. and Roberts, T. R., (eds.), Cambridge: Royal Society of Chemistry, pp. 411–420, 1999). Physical control measures (such as solarization and hot water treatment), biological control measures (e.g., crop rotation), and integrated approaches have been used to ameliorate the damage caused by plant parasitic nematodes (Whitehead, A. G., Plant Nematode Control, Wallingford: CAB International, 1998), but no single method or combination of measures is uniformly effective.
Because of the rapidly increasing limitations of existing products, there is a need for innovation in anthelminthic discovery. There exists an urgent need for new agents active against pathogenic and parasitic nematode species, e.g., compounds active against animal or plant parasitic nematodes. To facilitate the discovery of new anti-nematode compounds, there exists a need for the identification and validation of additional biological targets (e.g., nematode genes and proteins) against which such compounds can be directed. Furthermore, there exists a need for the development of new methodologies and screening technologies for the identification of compounds active against nematodes. In particular, there exists a need for the development of screening assays that can be conveniently performed in a high throughput format.